Kinetics of CO/CO2 and H2/H2O reactions at Ni-based and ceria-based solid-oxide-cell electrodes

Abstract

The solid oxide electrochemical cell (SOC) is an energy conversion technology that can be operated reversibly, to efficiently convert chemical fuels to electricity (fuel cell mode) as well as to store electricity as chemical fuels (electrolysis mode). The SOC fuel-electrode carries out the electrochemical reactions CO₂ + 2e⁻ ↔ CO + O²⁻ and H₂O + 2e⁻ ↔ H₂ + O²⁻, for which the electrocatalytic activities of different electrodes differ considerably. The relative activities in CO/CO₂ and H₂/H₂O and the nature of the differences are not well studied, even for the most common fuel-electrode material, a composite of nickel and yttria/scandia stabilized zirconia (Ni–SZ). Ni–SZ is known to be more active for H₂/H₂O than for CO/CO₂ reactions, but the reported relative activity varies widely. Here we compare AC impedance and DC current–overpotential data measured in the two gas environments for several different electrodes comprised of Ni–SZ, Gd-doped CeO₂ (CGO), and CGO nanoparticles coating Nb-doped SrTiO₃ backbones (CGOn/STN). 2D model and 3D porous electrode geometries are employed to investigate the influence of microstructure, gas diffusion and impurities.

Comparing model and porous Ni–SZ electrodes, the ratio of electrode polarization resistance in CO/CO₂vs. H₂/H₂O decreases from 33 to 2. Experiments and modelling suggest that the ratio decreases due to a lower concentration of impurities blocking the three phase boundary and due to the nature of the reaction zone extension into the porous electrode thickness. Besides showing higher activity for H₂/H₂O reactions than CO/CO₂ reactions, the Ni/SZ interface is more active for oxidation than reduction. On the other hand, we find the opposite behaviour in both cases for CGOn/STN model electrodes, reporting for the first time a higher electrocatalytic activity of CGO nanoparticles for CO/CO₂ than for H₂/H₂O reactions in the absence of gas diffusion limitations. We propose that enhanced surface reduction at the CGOn/gas two phase boundary in CO/CO₂ and in cathodic polarization can explain why the highest reaction rate is obtained for CO₂ electrolysis. Large differences observed between model electrode kinetics and porous electrode kinetics are discussed.